if your ailerons have suficient travel at low speed to control the plane and the airplane refuse to turn at hig speed this can only mean the ailerons doesn't move at hig speed, if ailerons doesn't move at hig speed this mean your ailerons linkages are not strong enough and bend ( if you used the linkages provided in box they should be ok !!!, lt40 is certainly not a jet and bending the piano wires provided will make you wing broken in flight first..)
So my suggestion is to look at your servo, is your servo strong enough?